This invention relates to chemical or electrochemical sensors based on Si FET technology and particularly relates to the construction of such devices for the measurement of hydrogen ions (pH) and other ion activity in solution. Such devices are known as ISFET's (ion selective field-effect transistors) and are similar to conventional insulated gate FET's except that the metal gate electrode is removed and the gate region is covered by a suitable insulator film. This insulating layer acts as the ion selective, sensitive membrane.
In its use as a pH sensor, the ISFET must be immersed in an electrolyte which, from the standpoint of the normal electrical connections made to the front side, is a very hostile environment. It is, therefore, desirable to design the ISFET so as to minimize contact between the electrical circuitry and the electrolyte solution so that the integrity, stability, and reliability of the device is maintained.
In the past many attempts have been made to achieve the desired isolation between the environment to which the ISFET is exposed and its associated electrical circuitry. These attempts have generally been in biomedical applications and have involved approaches such as locating the metallized source and drain bonding pads several millimeters from the gate region with electrical access being accomplished through diffused silicon conductors. In those devices the metallized surface forming the pads is protected from the environment by the use of epoxies or silicone rubber. This approach typically results in a long, slender probe which may be 150 microns thick by 500 microns wide by 600 microns long. While such probes may be suited for medical applications they have disadvantages in laboratory and industrial applications, for they present higher lead impedances due to the relatively long path of the conductors leading from the gate region. Also, they do not have the durability and reliability necessary to operate under the more extreme environments encountered outside the medical field.
As indicated above, the prior art devices typically use contacts on the front side (the chemically or electrochemically responsive side) of the ISFET. Thus, protection is required for the circuit leads or circuit elements to isolate them from the environment. In some cases this protection has been attempted by providing contact from the back of the device. This has been done, for example, by making contact with the source and drain by way of holes which have been laser machined all the way through the device or by migrating aluminum through the device. These methods all have the disadvantage of producing either a disrupted front surface or a region which is not mechanically strong. In this connection, it should be pointed out that a planar front surface is desirable in order to avoid susceptibility to contamination which can cause a fouling of the gate membrane.
It is an object of this invention to provide a method and means for providing an ISFET probe having protected contacts while maintaining a planar front surface which is mechanically strong. More particularly, it is an object of this invention to provide a method for producing back contacts for the source and drain regions of an ISFET probe which introduce no mechanical or electrical problems affecting the stability and durability of the probe.